Methods for coating various substrates are known, e.g., conventional casting technique, spray technique, etc. Rotational casting techniques have been employed for coating polyurethane elastomer compositions onto rigid substrates. Several advantages are associated with this method over the other known coating methods, for example, the rotational casting method requires shorter production time, there is no need for a mold to hold and composition while it cures, and loss of materials by, e.g., overspraying generally occurs is avoided.
Ruprecht et al., “Roll Covering by Rotational Casting with Fast-reacting PUR Systems”, Polyurethanes World Congress 1991 (Sep. 24-26) pp. 478-481, describes rotational casting techniques useful for producing roll coverings using fast-reacting polyurethane elastomer systems. In these systems, the polyurethane reaction mixture is metered through a movable mixing head which travels at constant speed in the axial direction along the rotating roll core a short distance above its surface. The polyurethane reaction mixture solidifies very quickly (in a matter of seconds), to produce a polyurethane coating with a thickness buildup of 4 to 5 mm. Additional layers of the polyurethane reaction mixture are applied until the desired thickness of polyurethane coating is achieved.
Rotational casting reduces the number of steps involved in roll coating. However, if the polyurethane reaction mixture gels too slowly, the polyurethane coating will drip off the roll. If the polyurethane reaction mixture is formulated to gel quicker, the polyurethane can gel in the head of the mixer or ridges can develop on the outer surface of the roll requiring machine finishing of the polyurethane coating on the surface of the roll.
U.S. Pat. No. 5,895,806, incorporated herein by reference, discloses a polyurethane prepolymer composition containing dual thixotropic agents and U.S. Pat. No. 5,895,609, also incorporated herein by reference, discloses a rotational casting method for coating a cylindrical object employing the polyurethane prepolymer composition of the '806 patent. Using the composition containing dual thixotropic agents, a thicker coating was achieved per each pass without any dripping or ridging. These polyurethane coating compositions have found wide commercial use on rigid substrates, e.g., metals, plastics and composites, in areas such as, for example, paper and mill rolls, industrial rolls and graphic art printing rolls.
U.S. Pat. No. 6,747,117, incorporated herein by reference, discloses a composition useful for rotationally casting cylindrical parts comprising an isocyanate-terminated polyurethane prepolymer and a curative agent comprising a polyaspartic ester, typically as part of a co-curative system along with compounds selected from aromatic diamines and diols.
U.S. Pat. No. 5,601,881, incorporated herein by reference, discloses the use of sheet die to allow even thicker coatings to be applied. The die is disposed parallel to the axis of rotation of the body at an angle alpha, whereby the rate of reaction of the reaction mixture and the relative movement are synchronized with the circumferential speed of the rotating body in such a way that the successive convolutions overlay in the form of scales and connect together seamlessly.
However, problems are associated with the use of a sheet die. For example, a sheet die that produces a film of material with length to width ratio of 10 to 300, has difficulty maintaining even flow rate across the outlet. Differences in flow can come from edge effects, because the edges of the die have more resistance to flow, or from channeling of flow in various areas. Channeling can come from partial plugging, or viscosity increase in one area of the die, resulting in the lower viscosity, fresher material taking the path of least resistance around this area. As a result, flow becomes less even and production must eventually be stopped to clean the die. This can be a serious problem, for example, when producing large rolls where the highest quality is demanded, such as in paper mill rolls. Also, starting and stopping the roll covering operation can result in a noticeable defect in the surface of the roll at the point that production was stopped, further resulting in the roll covering being scrapped.
US Pat Pub 2004/0091617, incorporated herein by reference, discloses a rotational casting method and device for producing thicker layers, but with wider processing windows, allowing for longer runs, lower scrap rates, and higher quality articles. The method comprises: (a) rotating the substrate about an axis at a selected rotational speed, (b) applying a polymeric reaction mixture to a surface of the rotating substrate by ejecting the polymeric reaction mixture through a die at a selected flow rate, said die dividing an inlet stream of the polymeric reaction mixture into plural outlet streams, the outlet streams being applied to the substrate and the outlet streams being spaced apart from each other such that the outlet streams flow together seamlessly after application to the substrate, (c) effecting relative linear movement between the rotating substrate and the die in a direction parallel to the axis of rotation at a selected relative linear speed, and, (d) synchronizing the reaction mixture flow rate, the relative linear speed and the rotational speed in such a way that successive convolutions of the outlet streams of the polymeric reaction mixture overlap and meld together seamlessly.
Despite the many rotational casting applications currently in use, the special rheological requirements on the polyurethane reaction mixture metered onto the substrate can place some restriction on the exact resin composition of the final coating, and hence the physical properties thereof. There is therefore a need for polyurethane reaction mixtures, i.e., compositions comprising polyurethane prepolymers and curing agents, that can expand the palette of polyurethane resins available for deposition by rotational casting.
Prepolymer compositions, useful in rotational casting systems, which employ a prepolymer formed from a select polyol and an organic diisocyanate monomer such as 4,4′-diisocyanato diphenylmethane (MDI), and a curing agent formulated with thixotropic reagents to control the rhological properties of the system as it is applied are known. These MDI prepolymers contain unreacted free MDI, which is a health hazard when present in the environment at certain levels. Some commercial rotational casting grades of prepolymer contain over 10% free MDI. US Pat Pub 2009/0076239 discloses MDI prepolymers with extremely low levels of MDI, e.g., less than 1% by weight.
It has been found that compositions comprising MDI prepolymers with low free MDI content and curatives of known rotational casting formulations not only improve the industrial hygiene associated with rotational casting, but also surprisingly provide polyurethane resins and products with improved mechanical properties.